fix netlify-cms and sass plugin css extraction

[erquhart]

Creates a separate ExtractTextPlugin instance for both the Netlify CMS and Sass plugins. We probably need every plugin that uses css extraction to do this.

Fixes #4335.

[gatsbybot]

Deploy preview for gatsbygram ready!

Built with commit 6e2fb06

https://deploy-preview-4379--gatsbygram.netlify.com

[KyleAMathews]

TravisCI error

[erquhart]

Hmm doesn't seem related.

[erquhart]

Seeing the same failure locally on master.

[erquhart]

@KyleAMathews failure is to due to #4378, it was merged before tests finished.

[KyleAMathews]

Summary of all failing tests
 FAIL  packages/gatsby-plugin-sass/src/__tests__/gatsby-node.js
  ● gatsby-plugin-sass › encountered a declaration exception
    TypeError: ExtractTextPlugin is not a constructor
      2 | const { cssModulesConfig } = require(`gatsby-1-config-css-modules`)
      3 | 
    > 4 | const extractSass = new ExtractTextPlugin(`styles.css`, { allChunks: true })
      5 | 
      6 | exports.modifyWebpackConfig = ({ config, stage }, options) => {
      7 |   const sassFiles = /\.s[ac]ss$/
      
      at Object.<anonymous> (packages/gatsby-plugin-sass/src/gatsby-node.js:4:21)
      at Suite.Object.<anonymous>.describe (packages/gatsby-plugin-sass/src/__tests__/gatsby-node.js:10:35)
      at Object.<anonymous> (packages/gatsby-plugin-sass/src/__tests__/gatsby-node.js:1:41)

^^^ is what I'm seeing on TravisCI

[KyleAMathews]

master could have a different problem /cc @m-allanson

[m-allanson]

master on Travis should be green now, see https://travis-ci.org/gatsbyjs/gatsby/builds

[erquhart]

That's the build, but the test fails.

[erquhart]

That sass plugin error is really strange, the code is definitely correct. I'll take another look in the morning.

[m-allanson]

That's the build, but the test fails.

Oh I see, yep! @erquhart Not that it helps, but I had a poke about at this locally. Like you say, it's strange as it looks like it should work 😕

[erquhart]

@KyleAMathews didn't realize the sass plugin test was mocking the extract plugin module, fixed now.

[KyleAMathews]

Oops, missed merging this a couple of days ago! Thanks!